Wind turbine generator having integrator tracking

ABSTRACT

A power generating system includes a wind turbine driven generator, the wind turbine having a wind driven rotor with a plurality of variable pitch angle blades. The blade angle is scheduled during acceleration and deceleration of the wind turbine by open loop controls to minimize stresses, and is scheduled during powered operation by closed loop controls to maintain desired torque or speed. The closed loop controls contain an integrator which produces an integral blade angle control signal. The scheduled blade angle is fed back to the integrators through an integrator tracking network to maintain the integral blade angle control signal at all times within a preselected range relative to the scheduled blade angle.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to the use of wind energy to drive a wind turbinefor the production of electrical power, and specifically to a controlsystem which automatically modulates the pitch angle of the wind turbineblades in order to regulate either electrical output power, shaft torqueor speed in order to minimize the effects of wind gusts and turbulence,and to reduce stress on the blades and other mechanical components.

2. Description of the Prior Art

Attempts to harness the forces of nature for man's benefit is recordedin the earliest pages of history. One of the first practicalapplications was the use of wind energy to drive windmills in order toproduce power. A concern that the world's available energy resourceswill eventually be depleted has resulted in renewed interest in thegeneration of power from naturally occurring phenomena, and has givenrise to the development of various schemes to generate this powereconomically, efficiently and dependably. As a consequence, the windmillhas received considerable attention as a partial solution to supplyingthe world's increasing energy demands.

The basic problem with windmill or wind turbine generated power is notits overall availability, but in harnessing this power in an efficientmanner and supplying it in the proper form useful to electricalutilities or to isolated stations. In many locations winds are, at best,unpredictable as to direction and velocity, and the availability ofuseful output power at any given time is uncertain. The amount of poweravailable varies with wind speed, and gusts cause transient changes inoutput power. While the windmill power output may be used directly todrive mechanical devices, its most useful form is electrical, in whichform the power may be transmitted to new or existing power grids for useby industry and homes. To produce useful electrical power, the rotaryenergy of the windmill is used to drive a dynamo, which produces a.c. ord.c. electrical power as desired. In some applications d.c. output poweris used to charge large storage batteries, the output from the storagebatteries being used to provide electrical power when needed. The use ofstorage batteries generally necessitates conversion from d.c. to a.c.via static inverters or other means. If a.c. power is produced ratherthan d.c. via a windmill driven synchronous generator, both thefrequency and phase of the a.c. power must generally be regulated, aswell as the power output, before the a.c. power can be transmitted tocommercial users, or fed into existing power grids.

It has been found that the control necessary to produce electrical powerfrom a synchronous generator, driven by a wind turbine, can be providedby varying the pitch angle of the wind turbine blades, in a manneranalogous to the blade pitch control for an aircraft propeller. U.S.Pat. No. 2,363,850 describes a wind turbine driven a.c. generator with aspeed governor controlled mechanism for varying the angle of the windturbine blades between fully feathered and fully powered positions.Means are described to regulate electrical output frequency, phase andpower, and to disconnect the electrical generator at wind velocitieswhich are too high or too low to produce the desired power. U.S. Pat.No. 2,547,636 provides an automatic speed control for a wind turbine tocontrol the charging rate of a storage battery, the speed controlconsisting of mechanical devices responsive to wind velocity forchanging the blade pitch angle. U.S. Pat. No. 2,583,369 is a similarcontrol for mechanically adjusting blade pitch angle to maintain arelatively constant electrical generator speed, and hence a relativelyconstant a.c. output frequency.

U.S. Pat. No. 2,795,285 is directed to a control for varying the rate ofchange of load, speed or voltage of a wind driven motor by varying thepitch of the wind turbine blades in a closed loop manner. U.S. Pat. No.2,832,895 is another device for adjusting the blade pitch of a windturbine in response to a predetermined charge on a battery, or inresponse to sudden gusts of wind.

The basic problem with the prior art devices is that they do not actrapidly enough, or with sufficient accuracy, to limit stresses in bladesand other mechanical components to acceptable levels. They are undulyaffected by wind gusts and turbulence, and cannot maintain satisfactorypower control over a wide range of wind conditions to allow connectionto a conventional power grid or power distribution system. At high windvelocities even mild turbulence creates significant fluctuations inpower, and may cause the generator to be removed from the power grid.

The present invention overcomes the limitations of the prior art, andprovides a very responsive and rapidly acting pitch control for theblades of a wind turbine. The control maintains a.c. electricalfrequency, phase, speed, torque and power within desired tolerances, andalso schedules the blade pitch angle during start up and shutdown toprevent undesired loads on the mechanical components. The control isadaptive in that the blade angle controls are responsive to windvelocity magnitude and to changes in wind velocity to maintainsatisfactory power, torque and speed control. The control system ispreferably electronic and is therefore fast acting, and may beimplemented inexpensively with digital computers or microprocessingequipment.

It is therefore an object of the present invention to provide animproved pitch angle control for a wind turbine which modulates the windturbine blade angle in response to a large number of operatingconditions.

Another object of the present invention is an electronic pitch anglecontrol for wind turbines which schedules blade angle to minimize bladestress and shaft torque variations during start up and shutdowntransients.

A further object of the present invention is an electronic pitch anglecontrol for wind turbines which regulates speed, torque and power outputof a turbine driven synchronous generator in a closed loop manner.

Another object of the present invention is a closed loop blade anglecontrol for a wind turbine in which proportional, integral andderivative control signals are produced, and in which the loop gains arecontinuously varied as a function of wind velocity.

A further object of the present invention is an electronic control formaintaining the a.c. output from a wind turbine driven synchronousgenerator at a predetermined power, frequency and phase and whichautomatically regulates the connection of the a.c. power into a grid.

Another object of the present invention is a power generating systemincluding a wind driven turbine which compensates the blade pitch anglecontrol for rapid changes in the wind.

A further object of this invention is a closed loop control for a windturbine driven generator in which the closed loop contains an integratorwhich automatically tracks the blade angle of the wind turbine even whenthe control is inactive.

Another object of this invention is a wind turbine driven generatorsystem in which wind speed is synthesized from system operatingparameters.

A further object of this invention is a wind turbine driven generator inwhich turbine blade angle is controlled as a function of eithergenerator speed or generator power depending on the connection of thegenerator to a power transmission grid.

SUMMARY OF THE INVENTION

In accordance with the present invention, a variable pitch wind turbineis connected via conventional gearing to drive a synchronous generatorin order to produce a.c. power which may be used directly to power aload, or fed to a conventional power grid system. During start up andshutdown of the wind turbine, the blade pitch angle is scheduled by openloop controls as a function of rotor speed and wind velocity. When thegenerator is operated independently of a power grid system, the bladepitch angle is scheduled by a closed loop rotor speed control, the loopcontaining proportional, integral and derivative control signals inaddition to lead compensation. When the generator is connected to apower grid system, the blade pitch angle is scheduled by a closed loopshaft torque or generator power control, the loop containingproportional, integral and derivative control signals. Under generatorpower or shaft torque control, the rotor speed control is modified toact as a topping or overspeed protection. The gains in the closed loopcontrols are continuously varied as a function of wind velocity tooptimize stability and transient response. Wind velocity may be senseddirectly, or synthesized as a function of system operating conditions.The control system for blade pitch angle is very responsive to windgusts and reacts rapidly via an anticipation control signal which issummed with the closed loop control signals during rapid changes in windconditions to minimize mechanical stresses. An integrator in the closedloop controls is forced via a feedback loop to track the blade pitchangle even when the closed loop controls are inactive. The controlsystem is specifically adapted for implementation using digitalelectronics, although analog electronic circuitry may also be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a representative wind turbine.

FIG. 2 is a schematic diagram showing the interrelationship between theturbine blades, the electrical generating system, and the blade pitchangle control system.

FIG. 3 is a schematic diagram of the blade pitch angle control system ofFIG. 2.

FIG. 4 is a graph showing the acceleration control schedule for theblade pitch angle.

FIG. 5 is a graph showing the deceleration control schedule for theblade pitch angle.

FIG. 6 is a schematic block diagram of the rotor speed control schedule.

FIG. 7 is a schematic diagram of the wind anticipation control schedule.

FIG. 8 is a schematic block diagram of the shaft torque controlschedule.

FIG. 9 is a schematic block diagram of the gain schedules for thecontrol loops of FIGS. 6 and 8.

FIG. 10 is a schematic diagram of a variation of the gain schedulingshown in FIG. 9 using synthesized wind velocity.

FIG. 11 is a schematic block diagram of alternative closed loop controlsfor blade pitch angle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is shown a representative wind turbineconstruction which consists of two diametrically opposed identical rotoror propeller blades 10, typically totaling 100 to 300 feet in diameter,mounted on an open truss tower 12 which provides adequate groundclearance for the blades while locating the blades in a relatively highwind velocity region. The rotor blades are generally made of aluminum,steel or fiberglass. The electrical generating and other mechanicalcomponents are contained in a nacelle 14 and mounted on a bed plate, notshown. The rotor blades 10 are located at the end of the nacelle 14downwind of the tower 12 to keep them from striking the tower whenflapping under impact loading. A yaw control, not shown, may be providedto rotate the nacelle 14 and maintain the rotor blades downwind inresponse to slow changes of a weather front, rather than permitting therotor blades to move freely about the yaw axis to follow sudden shiftsin wind direction. The nacelle 14 contains the hub for the rotor blades,a gearbox, a hydraulic pitch control for the rotor blades, a synchronousgenerator for producing electricity from rotation of the rotor blades,and the necessary gearing and controls.

In FIG. 2 there is shown the turbine blades 10 mounted on a hub 16, andthe electrical generating system and blade pitch angle control systemwhich are contained in the nacelle 14 of FIG. 1. In general, theelectrical generating system shown as a synchronous generator 26, andthe mechanical portion of the pitch control for the turbine blades 10,are well known in the art, and will not be described in detail.

A shaft 18 is connected at one end to the hub 16, and at the other endto a conventional gearbox 20 which steps up the rotary motion of thewind driven turbine in a ratio dependent upon the number of pairs ofpoles in the synchronous generator 26 and the desired output frequencyof the synchronous generator. In a typical installation, a wind turbinerotational speed of 40 rpm will be converted in the gearbox 20 to arotational speed of 1800 rpm to produce from a standard synchronousgenerator 60 cycle alternating current.

Output shaft 22 from the gearbox 20 is connected at its other end to thesynchronous generator 26. A conventional slip coupling may be insertedbetween the output shaft 22 and the synchronous generator. Thesynchronous generator 26 typically has a constant magnetic field and anarmature which delivers alternating current in synchronism with therotation of the armature, and at a frequency which is the product of thenumber of pairs of poles in the synchronous generator and the speed ofrotation in rpm. The electrical output from the generator 26 is fed viaa line 28, a switch 40 and a line 34 to a load, not shown. The output ofthe generator 26 may be single phase or three phase. The load may be astorage battery or other power storage device, in which case conversionto d.c. may be necessary, or the power may be supplied directly to aremote installation, in which case the frequency and phase of the outputpower may be critical. Typically, however, the a.c. power from thegenerator 26 is fed into the power grid of an electrical utility throughwhich it is transmitted via power transmission lines to a remotelocation. In this case, the phase relationship between the power gridand the generator output is quite critical, since the phase relationshipis a measure of the power transmitted from one to the other, and a phasemismatch between the output of the synchronous generator 26 and thepower transmission grid will not only reduce the efficiency of thesystem, but could in fact drain power from, rather than supply power to,the power transmission grid. Power variations can result in overheatingand damage to the synchronous generator. Consequently, there isconnected to the synchronous generator 26 an automatic frequency andphase synchronizer circuit 30, the construction of which is well knownin the art. The synchronizer circuit 30 insures that the frequency andphase of the synchronous generator is matched to that of the load orpower grid before the synchronous generator is connected "on line".Signals indicative of the frequency and phase of the synchronousgenerator output are fed to the synchronizer circuit 30 via signal line32. A signal line 36 also feeds to the synchronizer circuit 30 thefrequency and phase of the load or power grid appearing on line 34 withswitch 40 open. The automatic synchronizer circuit 30 compares thefrequency and phase of the synchronous generator with that of the load,and when synchronism occurs a discrete signal will be produced bysynchronizer circuit 30 on line 38 which will in turn close switch 40and connect the synchronous generator "on line". The discrete signal online 38 is also fed to the blade pitch angle control 46 to be describedsubsequently.

If the synchronous generator 26 is at the desired frequency but not inphase with the load, a signal is sent from the synchronizer circuit 30via signal line 42 to the blade pitch angle control 46 which will adjustthe rotor speed and therefore the frequency and phase of the output ofthe synchronous generator to produce synchronization as describedsubsequently in conjunction with FIG. 6.

Since the output frequency of the synchronous generator is controlled bythe speed of rotation of the wind turbine, the maintenance of apredetermined electrical output frequency such as 60 cps requiresprecise control of the wind turbine rotational speed. The most practicalmanner of controlling wind turbine rotational speed and thereforegenerator speed and output frequency, is to vary the pitch angle of therotor blade to prevent the wind turbine from speeding up when the windvelocity increases, or slowing down when the wind velocity decreases. Toprevent fluctuations in speed which occur with unpredictable wind gusts,the control for the blade angle changing mechanism must be veryresponsive.

In accordance with the present invention there is provided, as shown inFIG. 2, a pitch angle actuator 44 which comprises well known hydraulicactuators and linkages similar to those used with aircraft propellers,but on a larger scale. A control valve in the hydraulic portion of thepitch angle actuator 44 responds to an electrical signal from bladepitch angle control 46 transmitted via a signal line 48. The preferredcontrol valve in the pitch angle actuator 44 is a two stage hydraulicunit with a rapid slew rate, the valve moving the pitch angle controlmechanism through conventional linkages and levers. The signal on line48 is proportional to the blade angle error, β_(E), which represents thedifference between the desired or reference blade pitch angle, β_(R), asscheduled by the blade pitch angle control 46 (FIG. 3), and the actualblade pitch angle, β_(A). The actual blade pitch angle, β_(A), ismeasured by a transducer 50 located in the pitch angle actuator 44, andan electrical signal indicative thereof is fed via a signal line 52 tothe blade pitch angle control 46.

The blade pitch angle control 46 is used to modulate the pitch angle ofthe blades to minimize mechanical stresses during start up and shutdown,and during periods of gusting winds. It is also used as part of a closedloop control to regulate wind turbine rotor speed and thus electricaloutput frequency in one control mode, or generator output power or shafttorque in another control mode, depending on the type of load. Forexample, when the wind turbine and generator are used as an isolatedgenerating station, wind turbine rotor speed control is generallysufficient; however, when the generator is connected "on line" with apower grid shaft, torque or generator power control is necessary. Ineither case the control system must be responsive to wind turbulence tomaintain a reasonably constant generator output. The blade pitch anglecontrol 46 schedules the desired blade pitch angle, β_(R), in responseto selected operation conditions and reference signals, and providesrapid control of blade pitch angle from a fully feathered position,+90°, to a fully powered position, 0°. Since the rotor blades are notflat but have some twist, the pitch angle in degrees is referenced tothe pitch of the blade three quarters of the distance out along theblade from the hub.

In order to provide the necessary data to the blade pitch angle control46, the instantaneous rotational speed of the wind turbine rotor may besensed by a transducer 54 connected to hub 16, the transducer being, forexample, a toothed wheel having a magnetic pickup associated therewith,for providing an electrical signal, via line 56, proportional to therotor speed, N_(R). A similar type of transducer 58 may be connected tothe shaft in the synchronous generator 26 to provide an electricalsignal, via line 60, proportional to the speed of the synchronousgenerator N_(G). A transducer 62 such as a strain gage, or a pluralityof strain gages in different orientations, may be connected with a shaftin gearbox 20, or on shafts 18 or 22, to sense shaft torque Q and feed asignal proportional thereto, via line 64, to the blade angle control 46.The output power (or output current), P_(G), from synchronous generator26 may be measured and fed to the blade angle control 46 via signal line66 connected to the output line 28 of the synchronous generator. Othersignal transducers, amplifiers, and/or attenuators may be required, butare not shown for purposes of simplicity.

Also fed to the blade pitch angle control 46 are a plurality of fixed orvariable reference signal sources, which may be simple voltage levels inanalog format, or a digital word in digital format. A rotor speedreference signal, Nr REF, is generated in a block 69 and fed to thecontrol 46 via a line 70. A torque reference signal, Q REF, is generatedin a block 67 and fed to the control 46 via a line 68. A referencesignal indicated as ΔNr REF, which is used as a speed topping control,is generated in a block 71 and fed to the control 46 via a signal line72. A control signal indicated as FEATHER, used for feathering the windturbine rotor blade, is generated in a block 73 and fed to control 46via a signal line 74 and a switch 76.

The wind velocity is sensed by a wind velocity sensor 78, preferablymounted on the nacelle 14 of FIG. 1, or at some other location where itis not affected by the rotation of the wind turbine. The wind velocitysensor 78 measures instantaneous wind velocity, and feeds a signalindicative thereof via a signal line 80 to an averaging circuit 82, theaveraging circuit being an electronic integrator, or a digital ormicroelectronic component which provides statistical processing, andwhich determines average wind velocity over a preselected time. Theoutput from the averaging circuit 82, the average wind velocity, V_(WA),is fed to the control 46 via a signal line 84.

For purposes of the present exemplary description of the invention, itwill be assumed that the synchronous generator 26 begins to produceusable power at a wind velocity of 8 miles per hour and develops itsrated output, for example of 100 kilowatts, in a 18 mile per hour wind.It will also be assumed that the rated rotor speed is 40 rpm, at whichspeed an a.c. output of 60 cps is produced by the generator 26.

The details of a preferred implementation of the blade pitch anglecontrol 46 of FIG. 2 are shown in FIG. 3 in block diagram form. Thecontrol consists of an acceleration control schedule 86, a rotor speedcontrol schedule 88, a wind anticipation control schedule 90, a torquecontrol schedule 92, and a deceleration control schedule 94. Theoperation of the wind turbine can be separated into four operatingmodes, namely, start up, rotor speed control, torque (or power) controlwhen the wind turbine is connected to a power utility grid, and featheror shutdown. The control 46 provides an open loop scheduled control ofrotor blade pitch angle during start up and shutdown, and closed loopfeedback control of the rotor blade pitch angle for speed and torque (orpower) regulation. In addition, the gains in the rotor speed controlschedule 88 and the torque control schedule 92 are varied in response towind velocity by a gain schedule 95, via a signal line 97.

Each of the five schedules 86, 88, 90, 92 and 94 produces an outputsignal which is indicative of a desired blade pitch angle, and isreferred to as a blade angle reference signal, for the particularoperating conditions of the wind turbine. The output signal from theacceleration control schedule 86, an acceleration blade angle referencesignal, β_(S), appears on a signal line 96 and is fed as an input to amost select circuit 98. The output signal from the rotor speed controlschedule 88, a rotor speed control blade angle reference signal, β_(N),appears on a signal line 100 and is fed to a summing junction 102. Theoutput signal from the wind anticipation control schedule 90, a windanticipation blade angle reference signal, β_(ANT), appears on a signalline 104 and is also fed to summing junction 102 where it is summed withthe rotor speed control blade angle reference signal, β_(N). The outputfrom the summing junction 102, on a signal line 106, is thus the sum ofthe rotor speed control blade angle reference signal, β_(N), and thewind anticipation blade angle reference signal β_(ANT). The signal online 106 is also fed as an input to most select circuit 98. The outputsignal from the torque control schedule 92, a torque control blade anglereference signal, β_(Q), appears on a line 108, and is fed to a summingjunction 110 where it is also summed with the wind anticipation bladeangle reference signal, β_(ANT), on line 104, the output from thesumming junction 110 on a line 112 being fed as a third input to mostselect circuit 98.

The most select circuit 98 selects and passes therethrough the onesignal on lines 96, 106 or 112 which calls for the highest blade angle,i.e. that signal which schedules the blade angle closest to feather or90°. The selected signal during start up of the wind turbine willnormally be the acceleration blade angle reference signal, β_(S), online 96, and as the rotor speed increases and approaches the ratedspeed, the selected signal will normally be either the signal on line106 or the signal on line 112, depending on whether or not thesynchronous generator is connected "on line".

The output from the most select circuit 98 on a line 114 is fed as aninput to a summing junction 116. Also fed as an input to the summingjunction 116 is the FEATHER reference signal on line 74. If, however,switch 76 in line 74 is open, no signal appears on line 74, and theoutput from the summing junction on a line 118 is identical to that online 114, i.e., the output from most select circuit 98.

The output signal from the deceleration control schedule 94, adeceleration blade angle reference signal, β_(D), appears on a signalline 120, and is fed as the other input, together with the signal online 118, to a least select circuit 122. The least select circuitselects or passes therethrough the signal on input lines 118 or 120calling for the lowest blade angle, i.e., that closest to fully poweredor 0°. During normal powered operation the signal selected by the leastselect circuit will be that on the line 118. When, however, it isdesired to shut down the wind turbine rapidly, feather switch 76 isclosed and the FEATHER reference signal appears on line 74, this signalcalling for a very high blade angle. At this time the signal on line120, the deceleration blade angle reference signal, β_(D), will schedulea lower blade angle and will be the one selected by least select circuit122. Selection of the β_(D) signal permits the rate at which the bladeangle is feathered to be limited in order to minimize the stresses inthe blades when decelerating, and limiting the negative torque generatedby the rotor.

The output from the least select circuit on a line 124 is referred to asthe resultant blade angle reference signal, β_(R), and is fed to asumming junction 126 and compared with the actual blade pitch anglesignal β_(A), on line 52 to produce the blade angle error signal, β_(E),on line 48. It is this latter signal which is sent to the pitch angleactuator 44, FIG. 2.

The resultant blade angle reference signal, β_(R), on line 124 is alsoused for integrator tracking in the rotor speed control schedule 88, andin the torque control schedule 92, and is fed to both schedules via asignal line 128.

Each of the control schedules 86, 88, 90, 92 and 94 will be described indetail with reference to FIGS. 4-8.

To start the wind turbine, feather switch 76 is opened, removing theFEATHER reference signal on line 74. The signal β_(S) produced on line96 by the acceleration control schedule 86 is the selected signal atthis time, and is effective to vary the pitch angle of rotor blade 10 tomove it out of a feather position, +90°, at which there is no lift andtherefore no torque, and move it toward the fully powered position, 0°.As the speed of the rotor increases, the torque provided by the rotorincreases under certain conditions of pitch angle and rotor speed. Thereare some conditions of rotor speed and rotor pitch angle where negativetorque or deceleration occurs, so consequently the rate of pitch anglechange during start up is not arbitrary but must be scheduled inaccordance with the particular characteristics of the wind turbine. Ifthe pitch angle is changed too rapidly from the feathered position, therotor blade may stall. Consequently, a controlled or scheduled pitchangle change is required. Varying the pitch angle at a fixed rate fromthe blade feathered position until the wind turbine rotor reaches itsrated speed in one alternative which has been found useful, as long asthe pitch rate is varied rapidly in order to prevent the rotor bladefrom lingering at the rotational speeds which will excite the systemresonances. As wind speed increases, the time to start up the windturbine will decrease; a higher inertia rotor will take longer toaccelerate. Acceleration of the blade increases rapidly with rotationalvelocity.

While adequate performance during acceleration of the wind turbine maybe achieved by scheduling the change in rotor blade pitch angle fromfeather to fully powered at a fixed rate, considerably improvedperformance providing more rapid acceleration and reduced stress at allwind velocities may be achieved by scheduling blade pitch angle as afunction of average wind velocity, V_(WA), and rotor speed, N_(R). FIG.4 shows in graph form a bivariate acceleration control schedule in whichthe optimum acceleration blade angle is plotted with respect to windvelocity for different rotor speeds. Minimum starting blade angle isthus defined as a function of average wind velocity, V_(WA), and rotorspeed, N_(R). The schedule of FIG. 4 is implemented in accelerationcontrol schedule 86 of FIG. 3 in which the two input signals, V_(WA) andN_(R), appear respectively on signal lines 84 and 56, and the outputsignal on line 96 is the acceleration blade angle reference signal,β_(S), scheduled in accordance with the schedule of FIG. 4. Theimplementation is most easily accomplished digitally via a read onlymemory, although analog circuitry may be used. As may be seen in FIG. 4,at start up or 0 rpm, a blade angle of +70° or higher is scheduled,depending on wind velocity. As the rotor speed increases, providingtorque to the synchronous generator, the blade pitch angle is decreasedgradually toward 0° until the wind turbine rotor reaches its ratedspeed. The curves shown in FIG. 4 incorporate minimum blade angle limitswhich prevent the wind turbine rotor from generating accelerating torquewhich are greater than approximately 200% (or some other desiredlimiting value) of normal rated torque so as to minimize the bladestresses and torque transmitted through the rotor shaft and gearboxarrangement. Thus, during start up of the wind turbine, the rotor bladepitch angle is scheduled exclusively by the acceleration controlschedule 86.

While not shown in the Figures, the NR or rotor speed input signal tothe acceleration control schedule 86 of FIG. 3 can be replaced with onlyminor system variations by the N_(G) or generator speed signal, sincethere is a direct ratio between rotor rpm and generator rpm via thegearbox 20. The general shape of the curves of FIG. 4 will not change.

As the wind turbine rpm increases, in accordance with the accelerationcontrol schedule 86, the rotor rpm approaches the value scheduled in therotor speed control schedule 88, by the N_(R) REF signal on line 70.During start up, the actual rotor speed NR on line 56 will always beless than the desired rotor speed, NR REF, and the output from the rotorspeed control schedule on line 100, β_(N), will call for a low bladeangle, i.e. when underspeed is sensed, the rotor speed control schedule88 will schedule a low blade angle in order to attempt to increase therpm and bring the rotor and synchronous generator up to the desiredspeed. The most select circuit 98 at this time will not permit thesignal on line 106 to pass through, since a higher blade angle is beingcalled for by the β_(S) signal on line 96. As the rotor speed increasesand Nr approaches the value selected by the N_(R) REF signal, the β_(N)signal will call for a higher blade angle, while the β_(S) signal willcall for a lower blade angle, and a point is reached whereby control ofthe blade angle is assumed by the rotor speed control schedule 88.

Referring to FIG. 3, the rotor speed control schedule 88 is shown asbeing provided with input signals of desired rotor speed, N_(R) REF, online 70, actual rotor speed, N_(R), on line 56, and the topping speedreference signal ΔN_(R) REF, on line 72. Signals are also fed to therotor speed control schedule 88 from the automatic synchronizer cirucit30 (FIG. 2) via lines 38 and 42. Feedback of the blade angle referencesignal, β_(R), is provided via line 128, and gains for the controlschedule 88 are provided via line 97. Basically, the rotor speed controlschedule 88 compares the actual rotor speed, N_(R), with the desiredrotor speed, N_(R) REF, to produce a rotor speed error signal, fromwhich there is scheduled, through proportional, integral and derivativecontrols, the blade pitch angle, β_(N), to provide a stable, fastresponding system which minimizes excursions in rotor rpm, and thereforea.c. output frequency, which result from wind gusts or loss ofelectrical load. Rate of change of rotor speed is also monitored toprovide additional lead compensation. The topping signal, ΔN_(R) REF, isutilized only when the synchronous generator is connected on line. Thedetailed implementation of the rotor speed control schedule 88 is shownin FIG. 6.

Referring to FIG. 6, the actual rotor speed signal, N_(R), on line 56 iscompared with the desired rotor speed signal, N_(R) REF, on line 70, ina summing junction 130, and a speed error signal proportional to thedifference therebetween is produced on a signal line 132. The N_(R) REFsignal on line 70 may have added thereto, via a summing junction 131,the signal on line 42 to produce phase synchronization of the generator26 with the load as will be described. The N_(R) signal on line 56 isalso fed to a derivative circuit 134, and the output from derivativecircuit 134, a lead signal, is fed via a line 136 to a gain multipliercircuit 138. The gain of multiplier 138 is variable as a function ofwind velocity, and is shown as K_(N) on a signal line 97a. The variablegain feature of this invention will be described in conjunction withFIG. 9. The output from multiplier 138 is fed to summing junction 130via a signal line 142 in the same sense as the N_(R) signal, so that thesignal appearing on line 132 is acutally rotor speed error plus aconstant times the rate of change of rotor speed.

A switch 144 is provided in line 72, the switch being normally open,thereby preventing connection of the ΔN_(R) REF topping signal tosumming junction 130. When, however, the synchronous generator isconnected on line to a power grid, blade angle control is switched tothe torque schedule 92, (FIG. 3) and switch 144 is closed by thediscrete signal on line 38 to permit the ΔN_(R) REF signal to be fed tosumming junction 130. The ΔN_(R) REF signal is of a magnitude and senseso as to add to the N_(R) REF signal, thereby raising the desiredgenerator speed to a value above the rated 1800 rpm, depending on themagnitude of the ΔN_(R) REF signal. Since, however, the ΔN_(R) REFsignal is utilized only when the synchronous generator is on line, andwhen blade angle control has been transferred to the torque controlschedule 92 of FIG. 3, the ΔN_(R) REF signal acts as an overspeedprotection.

A signal appears on line 42 when the synchronous generator 26 of FIG. 2is at the desired frequency but not in phase with the load, this signalbeing of a magnitude and direction to temporarily increase or decreasethe rotor reference speed N_(R) REF on line 70. The presence of thesignal on line 42, which is summed with the N_(R) REF signal in summingjunction 131, will slightly adjust the rotor speed until phasesynchronization is achieved, at which time the signal on line 42 willbecome zero.

Assuming that switch 144 is open, and no signal appears on line 42, thesignal on line 132, rotor speed error plus a constant times the rate ofchange of rotor speed, is then fed to proportional, integral andderivative controls which are combined to produce the β_(N) signal online 100. The proportional control comprises a gain multiplier 146,having a variable gain, K_(NP), scheduled via a signal line 97b. Theoutput from multiplier 146 is fed as one input to a summing junction150. The integral control comprises a multiplier 152 having a variablegain, K_(NI), scheduled via a line 97c, the output from multiplier 152being fed via a line 156 as one input to a summing junction 158. Theoutput from summing junction 158 is fed via a line 160 to an integratorcircuit 162, the output from which is then fed via a line 164 to summingjunction 150 where it is summed with the proportional control signal.

In accordance with another aspect of the invention, integrator trackingis used to keep the integrator 162 in an inactive control near theresultant reference blade angle β_(R). The integrator output on the line164 is fed to a summing junction 166 and compared with the β_(R)feedback signal on line 128. The output from the summing junction 166, ablade angle error signal, is fed via line 168 to a gain circuit 170having a deadband as shown in FIG. 6. The function of gain circuit 170is to force the integral control signal to track the reference bladeangle signal β_(R) only when the blade angle scheduled by the integrator162 in the rotor speed control differs substantially from the resultantreference blade angle β_(R). The deadband insures that no trackingoccurs when the scheduled blade angle on line 164 is close to thatprovided by the resultant reference blade angle β_(R). The output fromthe gain circuit 170 is fed via a line 172 to the summing junction 158to be summed with the integrator input on line 156, the signal on line172 being zero when the blade angle error is within the deadband, andbeing non-zero to add to or substract from the integrator input whenoutside the deadband.

The derivative control comprises a multiplier circuit 174 having avariable gain, K_(ND), scheduled via a signal line 97d, the output fromthe multplier circuit 174 being fed via a signal line 178 to aderivative circuit 180. The derivative circuit output signal is fed viaa line 182 to a summing junction 184 where it is combined with theintegral and proportional control signals from summing junction 150appearing on a line 186. The output from summing junction 184 is therotor speed control blade angle reference signal, β_(N), on line 100.

In some applications the derivative circuits 134 and 180 of FIG. 6 maynot be required, and these circuits may be eliminated, or the respectivegains K_(N) on line 97a and K_(ND) on line 97d may be reduced to zero.The need for lead compensation depends on the nature of the sensedvariable of operation.

The torque control schedule 92 (FIG. 3) is used to minimize rotor shafttorque variations and rotor blade stresses, due to wind gusts andturbulence, when the synchronous generator is connected to an electricalpower grid. The preferred schedule senses the torque Q, on the shaftconnecting the wind turbine with the synchronous generator as itsprimary control variable. The actual shaft torque signal, Q, appears onsignal line 64, and the desired operating torque signal, Q REF, appearson signal line 68. The torque control schedule 92 is similar to therotor speed control schedule 88 in that the actual torque Q, is comparedwith the desired torque, Q REF, and the resulting difference or errorsignal is used to modulate the rotor blade angle through proportionalplus integral plus derivative controls so as to provide a stable,rapidly responding control loop which minimizes the torque variations.The proper selection of control gains, in combination with a rapidlyacting pitch angle control mechanism, provides damping on the torsionalresonance resulting from the wind turbine inertia and the shaft springrate. The control gains are also selected to minimize the torqueexcursions resulting from wind gusts. Providing damping to the torsionalwind turbine rotor resonance helps to reduce blade stress, gearbox loadsand shaft torques, and permits the use of a faster responding torquecontrol loop. The output from the torque control schedule 92 is thetorque control blade angle reference signal, β_(Q), appearing on line108. Feedback of the blade angle reference, β_(R), is provided via line128, and gain scheduling is provided via line 97.

The torque control schedule 92 produces the torque control blade anglereference signal, β_(Q), on line 108 only when the synchronous generatoris "on line", i.e., only when the output frequency and phase of thesynchronous generators are synchronized with the power grid network,within the limits determined by the synchronizer circuit 30, FIG. 2. Atthe same time as the output from the synchronous generator is connectedon line, the rotor speed control schedule 88 is converted into a speedtopping or overspeed control. Hence, when the generator is connected online, the wind turbine will always be underspeed relative to the desiredrotor speed, N_(R) REF plus Δ N_(r) REF, and the rotor speed controlblade angle reference signal on line 100, β_(N), will call for a lowerblade angle to increase the rotor speed. The torque control schedule 92,will limit the blade angle to conform to the torque limitations of thesystem, and will under most circumstances call for a higher blade angle,i.e., a blade angle closer to feather. Since the most select circuit 98of FIG. 3 passes the highest blade angle signal, the torque controlblade angle reference signal, β_(Q), will be the one passedtherethrough, and the rotor speed control schedule 88 will be effectiveonly in emergency situations when the rotor overspeeds, at which timethe rotor speed control blade angle reference signal, β_(N), will be thesignal calling for the higher blade angle.

FIG. 8 shows the details of the torque control schedule 92 of FIG. 3.The measured torque signal, Q, on line 64, and the desired torquesignal, Q REF, on line 68 are compared at a summing junction 188 toproduce a torque error signal on a signal line 190. Proportional controlis provided by a multiplier 192 having a variable gain, K_(QP),scheduled via a line 97e. Integral control is provided by a multiplier196 having a variable gain K_(QI), scheduled via a line 97f. The outputfrom multiplier 196 is fed via a line 198 to a summing junction 200,with the output from summing junction 200 being fed via a line 202 to anintegrator 204. The integrator output is then fed via a line 206 to asumming junction 208 where it is added to the output from multiplier 192on a line 210. Integrator tracking is provided as in the rotor speedcontrol schedule (FIG. 6) by reference blade angle, β_(R), on line 128,which is compared with the integrator output signal at a summingjunction 212, the difference signal being fed via a line 214 through again circuit 216, and then via a line 218 to summing junction 200. Thegain circuit 216 has a deadband, as shown in FIG. 8, to track theintegrator 204 only when its output differs substantially from thereference blade angle, β_(R).

Derivative control is provided by a multiplier 220 having a variablegain, K_(QD), scheduled via a line 97g. The output from multiplier 220is fed via a signal line 224 to a derivative circuit 226, and the outputfrom the derivative circuit 226 is fed via a line 228 to a summingjunction 230 where it is combined with the proportional plus integraloutput from summing junction 208 on a signal line 232.

The output on line 108 from summing junction 230, the proportional plusintegral plus derivative torque control blade angle reference signal,β_(Q), passes through a switch 234. The switch 234 is closed, permittingthe reference blade angle signal β_(Q), to pass therethrough, only whenthe synchronous generator is connected on line, and a discrete signal isproduced by synchronizer circuit 30 (FIG. 3) on line 38. The signal onlines 38 closes switch 40 (FIG. 2), connecting the synchronous generatorto the power grid network, and at the same time closes switches 144(FIG. 6) and 234 (FIG. 8), converting the rotor speed control schedule88 into a speed topping control as previously described, and alsoconnecting the torque control schedule 92 into the system. If thesynchronous generator is disconnected from the power grid network forsome reason, or if the frequency and/or phase of the synchronousgenerator deviates from that of the power grid network, the discretesignal on line 38 is removed, opening switches 40, 144 and 234, and thewind turbine reverts to rotor speed control.

While connected on line, and thus with the blade angle under the controlof the torque control schedule 92, generator speed and thus outputfrequency are maintained reasonably constant by the power grid network.Once connected on line, the power grid network will tend to maintain thespeed of the synchronous generator at the grid network frequency, and inphase therewith. A reduced rotor shaft stiffness will reduce the springrate of the shaft connecting the wind turbine to the synchronousgenerator and help to reduce shaft torque disturbances.

In accordance with another aspect of the invention, further aid inreducing shaft torque excursions when on line, and in reducing speedexcursions when off line, is the wind anticipation control schedule 90of FIG. 3, shown in detail in FIG. 7. The wind anticipation controlschedule 90 produces a signal on line 104, β_(ANT), for rapidly changingwind conditions through a nominal schedule of wind turbine blade angle,block 236, as a function of wind velocity, V_(WA), which appears as aninput signal to block 236 on line 84, and a derivative overlayanticipatory circuit in block 237. The wind anticipation schedule inblock 236 is obtained by calculating the blade angle required to providea constant power output for different wind velocities, assuming that thegenerator speed is constant at the desired reference value. Two windanticipation schedules may be used, one derived for 100% power when online torque control is utilized, and the second derived for ≈ 0% powerwhen off line. In either case the β_(ANT) signal on line 104 is nonzeroonly during rapidly changing wind conditions. The signal is added to therotor speed control blade angle reference signal, β_(N), at summingjunction 102, and is also added to the torque control blade anglereference signal, β_(Q), at summing junction 110. As wind velocityvaries, the anticipatory signal, β_(ANT), schedules a change in bladeangle which minimizes transient excursions in rotor speed or torquewhich would result from wind gusts. However, severe wind gusts willgenerally vary the torque or speed sufficiently to cause the generatorto be taken off line until the proper frequency and phase arere-established.

Alternately, shaft torque excursions may be reduced by conventional slipcoupling between the wind turbine and the synchronous generator.

The use of proportional plus derivative plus integral control, whichcombines into an integrator with quadratic lead compensation, in boththe generator speed control and torque control schedules 88 and 92,significantly improves the stability and response of the control loops.Quadratic lead compensation introduces an underdamped anti-resonancebefore the primary system resonant frequency which provides additionalphase lead to permit higher system gains and crossover frequency,thereby providing faster responding and more accurate control. Filteringmay be necessary if the speed and/or torque sensors are too noisy.

The deceleration control schedule 94 of FIG. 3 is shown in graph form inFIG. 5. In the event the wind turbine must be shut down rapidly, it isimportant to limit the rate at which the blade angle is feathered tominimize the stresses developed in the blades when decelerating the windturbine. In accordance with the invention, this limit is provided byincorporating a maximum blade angle limit which is scheduled as afunction of average wind velocity, V_(WA), and rotor rpm, N_(R). Theschedule is shown in FIG. 5 for a typical wind turbine, the decelerationcontrol blade angle reference signal, β, being plotted versus windvelocity, V_(WA), for selected rotor speeds, N_(R). The schedule of FIG.5 is implemented in FIG. 3 in which input signals of average windvelocity, V_(WA), on line 84, and rotor rpm N_(R), on line 56, are fedto the deceleration control schedule 94, the schedule being typically ananalog or digital bivariate function generator which produces an outputsignal on line 120, β_(D), which is in turn fed as an input to leastselect circuit 122. During normal operation of the wind turbine, thedeceleration control schedule 94 will produce a blade angle referencesignal, β_(D), which calls for a blade angle larger than that scheduledby the rotor speed control schedule 88 or the torque control schedule92. Hence the signal passed through the least select circuit 122 will bethe one appearing on input line 118 which calls for the lowest bladeangle. If the wind decreases below the velocity required to generaterated power or speed, the signal on line 118 will call for an even lowerblade angle.

The deceleration control schedule will be in sole control of the bladepitch angle when it is desired to shut down the wind turbine, that is,when switch 76 in the feather signal line 74 is closed. By closingswitch 76, a FEATHER signal appears on line 74 and is fed to summingjunction 116, the FEATHER signal being of a magnitude to produce on line118 a signal calling for a very large blade angle regardless of thesignal on line 114, the other input to the summing junction 116. At thistime the signal on line 120 will always call for a lower blade anglethan the signal on line 118, and the least select circuit 122 will passtherethrough the deceleration blade angle reference signal, β_(D), online 120. The wind turbine will thus decelerate and stop in accordancewith the schedule of FIG. 5, which limits the negative torque generatedby the rotor to a value of about two times normal rated positive torque.

As with the acceleration control schedule 86, it is apparent thatgenerator rpm, N_(G), can be used in place of rotor rpm, N_(R), in thedeceleration schedule.

Feather switch 76 is shown as a manual switch which in fact operates asan on-off switch, since opening the switch will eliminate the FEATHERsignal and cause acceleration of the wind turbine. It is apparent thatfeather switch 76 may be closed automatically if fail-safe circuitry isincorporated in the wind turbine, such as overspeed, overpower, orovertemperature sensors, and if such circuitry is connected to switch 76to cause closure thereof if an unsafe condition of operation is sensed.Other mechanical features such as a shaft brake may be incorporated asis well known to those skilled in the art.

Because of the nonlinear aerodynamic performance of the wind turbine, itis desirable to vary the control gains in the closed loop controls inresponse to operating conditions to optimize stability and transientresponse. The preferred implementation is shown in FIG. 9 in which theaverage wind velocity, V_(WA), on line 84 (FIG. 1), is fed into the gainschedule 95 (FIG. 3) which contains a plurality of analog or digitalfunction generators, 238, 240, 242, 244, 246, 248 and 250, which in turnproduce output signals on lines 97b, 97c, 97d, 97a, 97e, 97f and 97grespectively, indicative of the scheduled control gains K_(NP), K_(NI),K_(ND), K_(N), K_(QP), K_(QI), and K_(QD) respectively. The controlgains are fed to the appropriate gain multipliers in FIGS. 6 and 8 toschedule the gain of the multipliers as a function of wind velocity. Thegain curves shown in the function generators of FIG. 9 are merelyillustrative, since the actual gains will depend on many factors andcannot be determined accurately without analysis of the specific designand components of the wind turbine.

As an alternative to scheduling the control gains as a function ofaverage wind velocity as in FIG. 9, the control gains may be scheduledas a function of a synthesized wind speed, which is a function ofturbine blade pitch angle and shaft torque. In accordance with thisaspect of the invention, a preferred implementation is shown in FIG. 10in which signals indicative of the actual blade angle, β_(A), and shafttorque, Q, on lines 52 and 64 (FIG. 1) are fed into the gain schedule 95to an analog or digital bivariate function generator 252 in which shafttorque, Q, is varied as a function of synthesized wind speed, V_(WS),for a plurality of rotor blade angles, β_(A). The output from thefunction generator 252, synthesized wind speed, V_(WS), appears on line260 and is fed to function generators 262, 264, which schedule controlgains K_(NP), K_(QD), on lines 97b and 97g respectively. The other fivegain function generators illustrated in FIG. 9 have been omitted fromFIG. 10 for simplicity. Again, the curves shown in the functiongenerators of FIG. 10 are merely illustrative, but are similar to thosedescribed in FIG. 9.

It is apparent that other combinations of wind velocity, blade pitchangle and shaft torque, as well as other parameters, can be utilized forscheduling the variable control gains. Most prior art control systemsuse constant gains in the control loops, but with the advent of low costdigital microcomputers, it is simple and inexpensive to incorporatevariable gains in the control system.

FIG. 11 shows a modification of the blade pitch angle control 46 of FIG.3, in which the rotor speed control schedule 88 has been replaced by agenerator speed control schedule 266, and the torque control schedule 92has been replaced by a generator power control schedule 268. In thegenerator speed control schedule, actual generator speed, N_(G), onsignal line 60, is compared with a desired generator speed, N_(G) REF,generated in a block 269 and appearing on a signal line 270, to producean error signal, and proportional plus integral plus derivative controlswith variable gains are included in the control schedule 266, in amanner similar to that described in conjunction with FIG. 6, to producea generator speed blade angle reference signal, β_(NG), on a signal line272. Speed topping is provided by adding to the N_(G) REF signal a ΔN_(G) REF signal generated in a block 273 and appearing on a line 274when the synchronous generator is connected on line. Integrator trackingis also provided as in FIG. 6 using the reference blade angle, β_(R), online 128. The variable gain inputs to block 266 are not shown.

The generator power control schedule shown in a block 268 of FIG. 11 issimilar in operation to the torque control schedule 92 of FIG. 8. Theoutput electrical power, P_(G), from the synchronous generator on line66, FIG. 1, (or alternatively the current output only), is compared witha desired generator power reference signal, P_(G) REF, generated in ablock 279 and appearing on a line 280, (or alternatively a desiredgenerator current reference signal), to produce an error signal, andproportional plus integral plus derivative controls, variable gains, andintegrator tracking, are provided similar to the control of FIG. 8, toproduce a generator power (or current) blade angle reference signal,β_(p), on a line 282. The addition of wind anticipation control bladeangle reference signal, β_(ANT), to the reference signals β_(NG) andβ_(P), and the use of switching to connect the generator power controlschedule into the system only when the generator is on line, may beimplemented in the manner previously described.

In applications where more electrical power is required than can besupplied by a single wind turbine, a plurality of wind turbines may beplaced in parallel, but the respective synchronous generators mustproduce the same electrical feequency and phase. Paralleling is notaccomplished by varying the magnitude of the generated voltages as ind.c. voltage generation, but by varying the power input to the windturbines by scheduling the rotor blade angle.

The operation of the control system has been described primarily withreference to block diagrams and function generators, without specificdescriptions of the construction thereof. It is apparent that thecontrol system may be constructed entirely in analog format, with thesignals on the various lines being voltage levels. It is also apparent,however, that the preferred implementation of the control system isdigital, using existing microprocessors and/or digital computers toperform the necessary control functions. In its digital form, conversionof the sensed parameter signals from analog to digital, and reconversionof the actuator control signals from digital to analog, may be required.

While the wind turbine control system has been described in itspreferred embodiment, and the best mode of implementation has beendisclosed, it is apparent that changes and modifications may be made tothe construction and arrangement of the system and components thereofwithout departing from the invention as hereinafter claimed.

I claim:
 1. In a power generating system including a wind turbine drivengenerator, said wind turbine having a wind driven rotor with a pluralityof variable pitch angle blades:first control means responsive to a firstcondition of operation of said system for producing as a functionthereof a first blade angle reference signal indicative of a desiredblade pitch angle, second control means responsive to a second conditionof operation of said system for producing as a function thereof a secondblade angle reference signal indicative of a desired blade pitch angle,said second control means including integrator means producing anintegral blade angle control signal, means for selecting one of saidfirst and second blade angle reference signals, actuator meansresponsive to the selected one of said first and second blade anglereference signals for varying the pitch angle of said blades in responsethereto, and feedback means responsive to the selected one of said firstand second blade angle reference signals for maintaining the integralblade angle control signal produced by said integrator means at a valuewithin a preselected range relative to said selected one of said bladeangle reference signals.
 2. A power generating system as in claim 1 inwhich said feedback means includes:comparator means for comparing saidintegral blade angle control signal with the selected one of said firstand second blade angle reference signals to produce a blade angle errorsignal, variable gain means having a deadband receiving said blade angleerror signal and attenuating said blade angle error signal when saidblade angle error signal is outside a preselected range determined bysaid deadband and passing said blade angle error signal therethroughwithout attenuation when said blade angle error signal is within saidpreselected range, and means for feeding the output from said variablegain means as an input to said integrator means.